This invention relates to a solid state imaging device, a signal processing method and a driving method therefor and a camera, and more particularly to an X-Y address type solid state imaging device as represented by an amplification type solid state imaging device and a signal processing method for the solid state imaging device as well as a camera which employs an X-Y address type solid state imaging device as an imaging device. The present invention further relates to an amplification type solid state imaging device wherein pixels themselves have an amplification function and signals of the pixels are outputted as voltages and a driving method for the amplification type solid state imaging device.
An X-Y address type solid state imaging device includes, as shown in FIG. 14, a pixel section 111 wherein a large number of pixels are arranged in rows and columns, a vertical scanning circuit 112 for successively selecting the rows of the pixel section 111, a horizontal scanning circuit 113 for successively selecting the columns of the pixel section 111, and an output circuit (charge detection circuit) 114 for outputting a signal. The vertical scanning circuit 112 and the horizontal scanning circuit 113 are each formed from, for example, a shift register and successively generate a vertical scanning (vertical selection) pulse signal φV and a horizontal scanning pulse signal φH one by one for each row and each column, respectively.
In this X-Y address type solid state imaging device, when the horizontal scanning circuit 113 holds signal charge of the pixels in capacitors and outputs the pixel signals of the capacitors to the output circuit 114 from a horizontal signal line via horizontal switches each formed from a MOS transistor, dispersions in threshold voltage Vth of the horizontal switches (MOS transistors) are superposed on the pixel signals, and they appear as vertical string-like fixed pattern noises on a screen, deteriorating the picture quality.
Here, a generation mechanism of vertical string-like fixed pattern noises generated from the horizontal scanning circuit 113 is described with reference to an equivalent circuit of a signal path for the nth column of the horizontal scanning circuit 113 shown in FIG. 15. As a presupposition of description, notice is taken here of a dispersion in threshold voltage Vth of a horizontal switch.
Referring to FIG. 15, a pixel signal held by a capacitor 121 flows as charge to a horizontal signal line 123 when a horizontal scanning pulse φHn is supplied from a horizontal shift register (not shown) to the gate electrode of a horizontal switch 122 so that the horizontal switch 122 is put into a connecting state, and is then demodulated into a voltage by and outputted from an output circuit 124. In this instance, if the threshold voltage Vth which defines the boundary between a disconnecting state and a connecting state of the horizontal switch 122 exhibits a dispersion for each of the horizontal switches of the individual columns, then charge represented by the product of the dispersion in threshold voltage Vth and a variation amount in capacitance generated between the horizontal switch 122 and the horizontal signal line 123 appears on the horizontal signal line 123. Therefore, a vertical string-like fixed pattern noise which corresponds to the charge amount is superposed with the pixel signal.
The manner of such superposition is described with reference to FIGS. 16A and 16B which show equivalent circuits where the horizontal switch 122 is converted into a capacitance model. More particularly, FIG. 16A shows an equivalent circuit where the horizontal switch (MOS transistor) 122 is in a disconnecting (off) state, and FIG. 16B shows an equivalent circuit where the horizontal switch (MOS transistor) 122 is in a connecting (on) state.
In FIG. 16A, the horizontal switch 122 is in a disconnecting state and, as a capacitance model, a gate-drain capacitance 122a is produced between the gate electrode of a horizontal switch (MOS transistor) to which a horizontal scanning pulse φHn is applied and the capacitor 121 by which a pixel signal is held while a gate-source capacitance 122b is produced between the gate electrode of the horizontal switch and the horizontal signal line 123, and the capacitor 121 and the horizontal signal line 123 are disconnected from each other.
On the other hand, in FIG. 16B, the horizontal switch 122 is in a connecting state and the capacitor 121 is connected to the horizontal signal line 123, and a gate-channel capacitance 122c is produced between the gate electrode of the horizontal switch (MOS transistor) to which a horizontal scanning pulse φHn is applied and the horizontal signal line 123. Here, the capacitance of the gate-channel capacitance 122c is considerably higher than the total capacitance of the capacitance 122a and the capacitance 122b. 
Since the two states of FIGS. 16A and 16B are changed over with reference to the threshold voltage Vth of the horizontal switch 122 by the voltage of the horizontal scanning pulse φHn applied to the gate electrode of the horizontal switch 122, if the horizontal switch of each column has a dispersion in threshold voltage Vth, then the product of the dispersion in threshold voltage Vth and the difference between the capacitances of the horizontal switch 122 in the two states of FIGS. 16A and 16B appears as dispersion charge on the horizontal signal line 123 and makes a vertical string-like fixed pattern noise.
Now, where the capacitances of the capacitance 122a, capacitance 122b and capacitance 122c are represented by Cdg, Cgs and Cg, respectively, the dispersion of the threshold voltage Vth of the horizontal switch 122 is represented by ΔVth, the dispersion charge appearing on the horizontal signal line 123 is represented by Δq, the capacitance of a detection capacitor 125 of the output circuit 124 is represented by Cd, and a vertical string-like fixed pattern noise appearing on the output is represented by ΔVout, the dispersion charge Δq and the fixed pattern noise ΔVout are given byΔq=(Cg−Cgd−Cgs)·ΔVthΔVout=Δq/Cd
Particularly, giving an example of numerical values, if Cgd and Cgs are 1 fF, Cg is 20 fF, the dispersion ΔVth of the threshold voltage Vth is 50 mV and the capacitance Cd of the detection capacitor 125 is 0.5 pF, then the fixed pattern noise ΔVout is 1.8 mV.
Driving timings of the ordinary X-Y address type solid state imaging device and a manner in which vertical string-like fixed pattern noises appear are illustrated in a timing chart of FIG. 17. A vertical scanning pulse signal φV (φV1, . . . , φVm, φVm+1, . . . ) for selecting pixel elements of the same row successively rises for each horizontal blanking period, and an operation pulse signal φOP rises in synchronism with the vertical scanning pulse signal φV. The operation pulse signal φOP is applied to the gate electrode of an operation switch (not shown) formed from a MOS transistor for reading out a pixel signal to the capacitor 121.
As the operation pulse signal φOP rises, pixel signals of a selected row are read out into the capacitors 121. The pixel signals of the certain row held in the capacitors 121 are, when a horizontal image period is entered, read out from the output circuits 124 as the horizontal switches 122 are successive put into a connecting state when the horizontal scanning pulse signal φH (φH1, . . . , φHn, φHn+1, . . . ) outputted from the horizontal shift register successively rises.
In this instance, if it is assumed, for example, that an equal signal amount is outputted from all pixels and only the threshold voltages Vth of the horizontal switches 122 have individual dispersions, then as seen in the timing chart of FIG. 17, an output signal OUT does not exhibit an equal signal amount, but exhibits dispersions in threshold voltage Vth of the horizontal switches 122 superposed on the pixel signals. Then, the dispersions appear as vertical string-like fixed pattern noises on the screen, deteriorating the picture quality.
As a method of preventing deterioration of the picture quality arising from vertical string-like fixed pattern noises, a possible method is to extract only fixed pattern noise components, hold them as a reference signal for cancellation and subtract, in an ordinary imaging operation, the reference signal from signal outputs of the solid state imaging device to cancel the fixed pattern noises.
However, while, in the description of the generation mechanism of fixed pattern noises given above, a manner in which fixed pattern noises appear on an output signal in the condition that no incident light is received is described, if light is irradiated upon a central portion of the imaging area here, then signal components arising from the incident light are added to the fixed pattern noise components, and such an output signal waveform as indicated by OUT-L in FIG. 17 is obtained. This output signal cannot be used as a reference signal for cancellation.
In other words, in a conventional X-Y address type solid state imaging device, in order to cancel vertical string-like fixed pattern noises, the solid state imaging device must be shielded against incident light by some method so as to output only fixed pattern noise components as a reference for a correction signal. More particularly, such a mechanical operation that a cover is fitted on the lens of the camera or incident light is intercepted by a mechanical shutter is required. Such an operation is disadvantageous in terms of the price or minimization of a camera since it urges a person who operates the camera to perform a manual operation for cancellation of fixed pattern noises or requires a part which is not originally necessitated for a camera such as a mechanical shutter.
Meanwhile, as amplification type solid state imaging devices, a CMD (Charge Modulation Device), a BASIS (Base Stored Image Sensor), a BCMD (Bulk Charge Modulation Device) and so forth are known. In those amplification type solid state imaging devices, since pixels are formed using an active element of a MOS structure or the like in order to make the pixels themselves have an amplification function, a dispersion in characteristic (threshold value Vth and so forth) of an active element is superposed as it is on an image signal. Since the dispersion in characteristic has a fixed value for each pixel, it appears as a fixed pattern noise (FPN) on a screen.
An exemplary one of conventional amplification type solid state imaging devices constructed so as to remove fixed pattern noises arising from characteristic dispersions of pixels is shown in FIG. 24. Referring to FIG. 24, a large number of pixels 301 are arranged in rows and columns, and control input terminals of the pixels 301 are individually connected to vertical selection lines 302 in units of a row while output terminals of the pixels 301 are connected to vertical signal lines 303 in units of a column. Terminals of the vertical selection lines 302 on one side are connected to output terminals of a vertical scanning circuit 304 for the individual rows. The vertical scanning circuit 304 is formed from a shift register or a like element and successively outputs a vertical scanning pulse signal φV (φV1, . . . , φVm, φVm+1, . . . ).
Connected to each of the vertical signal lines 303 are the drains of two sampling switches 305s and 305n each formed from an N-channel MOS transistor. An operation pulse signal φOPS for sampling a signal voltage in a bright state prior to pixel resetting outputted from a pixel 301 is applied to the gates of the sampling switches 305s. Meanwhile, another operation pulse signal φOPN for sampling a signal voltage in a dark state after pixel resetting outputted from a pixel 301 is applied to the gates of the sampling switches 305n. 
The sources of the sampling switches 305s and 305n are connected to terminals of two capacitors 306s and 306n on one side, respectively. The capacitors 306s and 306n are provided to hold a signal voltage in a bright state and a signal voltage in a dark state, respectively, while the other terminals of them are grounded commonly. The sources of the sampling switches 305s and 305n are further connected to the drains of two horizontal selection switches 307s and 307n formed from N-channel MOS transistors, respectively.
The sources of the horizontal selection switches 307s and 307n are connected to a horizontal signal line 308, and the gates of the horizontal selection switches 307s and 307n are connected to output terminals of a horizontal scanning circuit 309 for the individual columns. The horizontal scanning circuit 309 is formed from a shift register or a like element and outputs a horizontal scanning pulse signal φH ( . . . , φHn, φHn+1, . . . ) for successively turning on the horizontal selection switches 307s and the horizontal selection switches 307n for the individual columns. The horizontal signal line 308 is connected to an input terminal of a horizontal output circuit 310. An output terminal of the horizontal output circuit 310 is connected to an input terminal of a CDS (correlation double sam-pling) circuit 311.
Subsequently, circuit operation of the conventional apparatus having the construction described above for removing fixed pattern noises is described.
If a certain row is selected by vertical scanning by the vertical scanning circuit 304 in a horizontal blanking period, then signal voltages in a bright state prior to pixel resetting and signal voltages in a dark state after pixel resetting of the pixels 301 of the selected row are successively sampled by the sampling switches 305s and 305n and held by the capacitors 306s and 306n, respectively.
Then, in a horizontal effective period, when a certain column is selected by horizontal scanning by the horizontal scanning circuit 309 and the horizontal selection switches 307s and 307n of the selected column are successively turned on, the signal voltages in a bright state and the signal voltages in a dark state held in the capacitors 306s and 306n are successively read out into the horizontal signal line 308, respectively. Consequently, the signal voltages in a bright state and the signal voltages in a dark state are successively transmitted in units of a column on a time base by the horizontal signal line 308 and supplied to the CDS circuit 311 through the horizontal output circuit 310.
By the CDS circuit 311, correlation double sampling of the signal voltages in a bright state and the signal voltages in a dark state which successively appear on the time base is performed, and finite differences between them are calculated to cancel noise components. As a result, a signal from which fixed pattern noises arising from dispersions in characteristic such as a threshold voltage Vth for the pixels 301 have been removed is obtained.
However, with the conventional amplification type solid state imaging device described above, while fixed pattern noises arising from characteristic dispersions of the pixels 301 can be removed, since the flows of signals in a bright state and a dark state are different in the sample hold circuit between the vertical signal lines 303 and the horizontal signal line 308, if some components are superposed on a signal by the sample hold circuit, then those components remain also after the correlation double sampling by the CDS circuit 311.
What are present as components which are superposed by the sample hold circuit are distribution noises of the sampling switches 305s and 305n and so forth. Where those components are different between columns because of dispersions in circuit characteristic, also the components which remain after the correlation double sampling exhibit dispersions and appear as vertical string-like fixed pattern noises on the screen.